In the art of treating substerranean formations to stimulate the recovery of fluids such as crude oil and gas, hydraulic fracturing of one or more fluid rich zones is widely practiced. Conventional hydraulic fracturing techniques suffer from several disadvantages, depending on the characteristics of the rock formation. In almost all cases the development of the fracture and the ultimate yield of fluids from the formation as a result of the fracture is limited by the inability to pump fluids down the wellbore and out through perforations in the well casing at a rate sufficient to overcome pipe friction losses and leak off of the fracturing fluid into the formation itself. Typically, the fracturing fluid pumping rate in many applications may not be sufficient to initiate and maintain a fracture for an adequate duration of time to accept a sufficient amount of propant carried in the fracturing fluid to open the fractures wide enough so as to produce satisfactory yields of well fluids.
In order to overcome the disadvantages and limitations of conventional surface pumping of subterranean formation fracturing fluids it has been proposed to place devices in the wellbore at various depths which will generate sufficient energy to propel a quantity of fracturing fluid into the formation. For example, U.S. Pat. No. 3,101,115 to M. B. Riordan, Jr. describes a well treating method and apparatus wherein a gas generator canister is lowered into a wellbore above a column of fluid in the wellbore and ignited to generate gases for propelling the liquid fracturing fluid into the formation to be fractured without interrupting the continuous delivery of fluid to the wellbore by surface pumps. However, the system and method contemplated by the Riordan, Jr. patent utilizes the gas generator only to boost the flow rate of conventional liquid well treating fluids momentarily and does not develop a preliminary "pad" of gas as part of the initial fracturing process and flowing ahead of a propant laden well treating fluid.
U.S. Pat. No. 4,039,030 to Godfrey et al contemplates the use of an explosive charge and a propellant generator in a wellbore wherein the propellant is detonated first followed by the detonation of a high explosive to maintain pressure of the high explosive over a longer period of time to extend the fractures caused by the explosive while pumping a fracturing fluid into the fractured formation.
An improvement in gas generating and injection devices for perforating a well casing at a production zone and initiating fractures with the production of a propellant gas is disclosed and claimed in U.S. Pat. No. 4,391,337 issued jointly to Franklin C. Ford, Gilman A. Hill and Coye T. Vincent. In this patent a combustion gas generator is provided in the form of a canister which may be suspended in the wellbore and is provided with a plurality of spaced apart shaped charge devices or grenades for perforating the well casing and contiguous layer of cement, if used, to provide apertures for the flow of gas and other fluids to be injected into a formation to be produced.
Accordingly, the prior art suggests the provision of downhole gas generators for use in fracturing operations which have not proven particularly attractive from an economic or technical viewpoint. In conventional hydraulic fracturing, even with the use of downhole propellant gas generators, a substantial amount of hydraulic power capability must be maintained at the surface in the form of large pumping capacity. The energy losses suffered in transmitting the hydraulic fluid through the well pipe or casing cannot be sufficiently overcome to provide the substantial volumes of fluid at pressures required to perform a suitable high stress fracture. Moreover, such prior art methods have not provided for an economical process able to generate suitable fracture initiation and entry into the fractures of a fluid that will satisfactorily open the fractures ahead of the entry of a propant laden fracturing fluid.